Boric oxide has many industrial applications, especially in the glass-making field where it is desirable to have a source of B.sub.2 O.sub.3 without the undesirable presence of sodium, such as is contributed by borax, or excess water to evaporate, such as from boric acid. Boric oxide is generally produced on an industrial scale by the dehydration of boric acid at high temperatures such as in the range of about 700.degree.-950.degree. C. in a glass furnace fired by oil or gas. The molten glass is solidified by allowing a continuous ribbon to flow over chill rolls and then crushed and screened to the desired particle size. See Kirk-Othmer, "Encyclopedia of Chemical Technology" Fourth Edition, Volume 4, Page 370, and U.S. Pat. No. 2,893,838. The product is a high purity (99% B.sub.2 O.sub.3) amorphous solid, but it has the disadvantage of being hygroscopic, tending to rehydrate and cake under normal use and storage conditions. Further, it is expensive to manufacture due to the energy required to maintain the furnace at the required high temperatures. A lower B.sub.2 O.sub.3 content product has been manufactured by reacting borax with sulfuric acid and then feeding the resultant mixture to a gas-fired furnace operating at 800.degree.-900.degree. C. The resultant amorphous product has a 95-98% B.sub.2 O.sub.3 content but also contains residual sodium, which is undesirable in many glass-making applications. The process also has high energy requirements which make the product expensive to produce. See Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Volume V, Part A, Pages 171-174, U.S. Pat. No. 3,468,627 and U.S. Pat. No. 3,479,137.
High-purity, crystalline boric oxide has also been produced such as by the slow dehydration of boric acid in an oven at 225.degree.-250.degree. C. over a period of from 7 to about 14 days. The use of sub-atmospheric pressures or organic liquid carders have also been suggested. See U.S. Pat. Nos. 2,137,058, 2,186,257, 3,397,954, 3,582,272 and 4,098,196, Italian Patent 467,440 [Chemical Abstracts 47,4563 (1953)], as well as the Kirk-Othmer and Mellor references discussed above, and Kemp, "The Chemistry of Borates," Part 1, Pages 10-11 (1956).
British Patent 1,278,466 describes a process for preparing a granulated bode oxide by heating granulated ortho-bodic acid in a gas diffusion apparatus, such as a porous plate through which is passed a stream of hot air, forming a fluidized bed. The product is a granulated boric oxide having a B.sub.2 O.sub.3 content approaching 98%.
Stanton U.S. Pat. No. 3,582,272 describes preparation of crystalline hexagonal boric oxide from boric acid in a heated dryer-agitator on a seed bed of crystalline boric oxide.